The present invention relates generally to materials that exhibit thermodynamically first-order magnetic transitions (i.e., the magnetic state change is accompanied by changes in the volume and/or crystal structure of the material) and, more particularly, to a method of enhancing the magnetocaloric effect (MCE) in such magnetostructural materials.
The magnetocaloric effect (MCE) describes the adiabatic conversion of a magnetically-induced entropy change to the evolution or absorption of heat, with a corresponding rise or decrease in temperature. FIG. 1 provides a schematic illustration of the magnetocaloric effect in a material 100. It is usual to quantify the MCE by the entropy change achieved by a change of magnetic field 102. The entropy change is determined from dc magnetic measurement by using the Maxwell relation:
                                          (                                          ∂                                  S                  ⁡                                      (                                          T                      ,                      H                                        )                                                                              ∂                H                                      )                    T                =                              (                                          ∂                                  M                  ⁡                                      (                                          T                      ,                      H                                        )                                                                              ∂                T                                      )                    H                                    [        1        ]            which can then yield a magnetic entropy change ΔSM:
                              Δ          ⁢                                          ⁢                                    S              M                        ⁡                          (                              T                ,                                  Δ                  ⁢                                                                          ⁢                  H                                            )                                      =                              ∫                          H              1                                      H              2                                ⁢                                                    (                                                      ∂                                          M                      ⁡                                              (                                                  T                          ,                          H                                                )                                                                                                  ∂                    T                                                  )                            H                        ⁢                                                  ⁢                                          ⅆ                H                            .                                                          [        2        ]            In the previous expressions [1] and [2], T is the absolute temperature and H is the magnetic field. Typical (adiabatic) magnetocaloric temperature changes under an applied field change of 7 T range from ΔTad≈2.5 K at T=10 K for Nd to ΔTad≈12 K at T˜180 K for Dy.
Systems employing the magnetocaloric effect are important for energy-efficient, low-CO2 emission refrigeration, air conditioning for vehicles and buildings, as well as for responsive temperature/heat sensor applications. Materials with a large magnetocaloric effect may also be utilized as heat pumps and, compared with the conventional vapor-cycle refrigerator, the magnetic refrigerator is environmentally benign and has a number of advantages which include high efficiency, low mechanical vibration and compact size.
Briefly, an active magnetic regenerator (AMR) refrigerator employs a porous or particulate bed of a magnetic refrigerant working material that acts as both the coolant that produces refrigeration and the regenerator for the heat transfer fluid. As the magnetic working material is subjected to the application of a magnetic field, the particles of the material warm in an adiabatic manner from the MCE and absorb heat from the environment. As fluid flows through the particle bed from the cold end to the hot end, the working material warms the fluid via heat transfer. The heat from the fluid is removed at the hot heat sink in the heat exchanger. After the fluid flow is stopped, the magnetic field is removed which then causes the magnetic working material to cool. The hot fluid is forced back to the now-cool porous bed of material where it is cooled by the bed. Remnant heat is removed from the fluid by the cold sink in the cold heat exchanger.
The potential applications of magnetic refrigeration are wide-ranging; with properly optimized performance, it is expected that they will be employed in building climate control, frozen food processing plants and supermarket chillers. Utilization of these materials can be envisioned in automotive and aircraft climate control, with an especially promising application of automotive climate control for zero-emission electric vehicles.
Magnetic refrigeration technology can accomplish those objectives in an environmentally-friendly manner, without the use of ozone-depleting chemicals such as CFCs (halogenated chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbons), PFCs (fluorocarbons) and SF6 (sulfur hexafluoride), other hazardous chemicals (NH3) and without the production of additional greenhouse gases. The energy efficiency resulting from use of technologies employing magnetic refrigeration is anticipated to reduce the amount of energy consumed as well as reduce CO2 emissions. Thus, two significant benefits of magnetic refrigeration technology are the replacing CFC's (which will reduce the potential for global warming) and designing climate control in large buildings and electric vehicles. The latter technology will allow a greater fraction of the available automobile power to be used for transportation rather than be exhausted for climate control.
With those advantages in mind, significant challenges to the technological development of these systems exist. Probably the most daunting technological hurdle remaining is the development of a cost-effective MCE material requiring practical magnetic fields and size considerations.
All magnetic materials, to a greater or lesser degree, exhibit a magnetocaloric effect. However, some materials, by virtue of a unique electronic structure or physical nanostructure, may display a significantly enhanced MCE which may potentially be harnessed for technological application. In contrast to the MCE found in paramagnetic (PM) materials, the large MCE exhibited by ferromagnetic (FM) materials near their magnetic phase transition temperature (the Curie temperature TC) renders them suitable as working materials for magnetic cooling at temperatures T>˜20 K, and up to the target temperatures appropriate for commercial, industrial and home refrigeration application and heat pump devices, 200 K–400 K (approximately −70° C.˜130° C.).
Further enhancements of the ferromagnetic MCE are possible if the magnetic transition at TC is accompanied by a crystallographic lattice distortion, as is often found in strongly-correlated electron systems. This lattice distortion may be either a contraction or an expansion of the atomic lattice, and may or may not include a change of symmetry. Such coupled magnetostructural changes are often referred to as first-order magnetic phase changes. The enhancement of the MCE in such materials arises because of the overall greater entropy change that occurs with a combined crystallographic and magnetic phase change. While materials such as amorphous and nanocrystalline alloys, intermetallic compounds, perovskite-type oxides and, recently, pnictides and carbides, have all been topics of vigorous study over the past 5 years by virtue of their interesting MCE properties, to date the most promising MCE material is the intermetallic compound series based on the composition Gd5(SixGe1-x)4, where 0≦x≦0.5.
The magnetocaloric effect produced by this family of compounds, also referred to as GdSiGe, has been christened as “giant” because of its extremely large magnitude. In particular, the MCE of the GdSiGe alloys is reversible and exceeds that of any other known magnetic material by a factor of two. Another noteworthy characteristic of the GdSiGe family is that the Curie temperature, and hence the MCE, may be tuned with compositional variation. This feature allows the working temperature of the magnetic refrigerator to vary from 30 K to 276 K, and possibly higher, by adjusting the Si:Ge ratio. Moreover, the GdSiGe series of compositions is “metamagnetic”, which means that the magnetic transition from ferromagnetic to paramagnetic behavior at the Curie point can be induced by applied field and pressure as well as by increased temperature.
However, magnetocaloric materials made from gadolinium (Gd) and in particular its alloys are generally very expensive and require very large and, therefore, impractical magnetic fields on the order of 2–10 T to yield a large magnetocaloric effect. For example, the typical AMR refrigerator described above, utilizing approx. 3 kg of Gd spheres, operates near room temperature in applied magnetic fields between 1.5 T and 5 T. This design provides a temperature span of 38 degrees for a field change of 5 T, and it generates up to 600 W of cooling power in a 5 T field with an efficiency that approaches 60% at 5 T. While these may be impressive figures, a magnetic field of 5 T can only be generated with a superconducting magnet that needs liquid helium to operate. Thus, the main drawback impeding the successful exploitation of the GdSiGe alloys is that the magnetic field magnitude required to obtain the spectacular magnetocaloric effect is simply too high for wide-spread commercial, home and transportation sector use.
U.S. Pat. No. 5,743,095 to Gschneidner, et al. discloses an improved Gd5(Si1-xGex)4 magnetic refrigerant that provides a high magnetocaloric effect and a high regenerator efficiency parameter. It is stated that the inclusion of a magnetically-soft alloying element, such as Mn, Fe, Co or Ni within the Gd5(Si1-xGex)4 compound optimizes the magnetocaloric effect properties of the refrigerant. However, the magnetic field necessary to influence the magnetocaloric effect of this improved compound is still obtainable only with liquid-He-cooled superconducting magnets, which are not practical additions in typical applications.
U.S. Pat. No. 4,985,072 to Sahashi, et al. discloses a composite material consisting of finely crystalline powders of magnetic rare-earth-based (Laves phase) intermetallic compounds that may be directly compacted in a metallic binder matrix or which may first be layered with Ni, Co or Fe and then compacted in a densely solid metallic binder matrix. There are two main motivations behind Sahashi et al.'s invention: i) to provide a plurality of compounds with different magnetic transition temperatures within a single compact and ii) to improve thermal conductivity of the magnetic substances. Sahashi et al. Claim their magnetic composite can provide a high magnetocaloric effect over a wide range of temperatures, and demonstrate magnetic transition temperature ranges of approximately 10 K to 70 K, depending upon the specific embodiment. The materials of the Sahashi teachings would, therefore, be unsuitable for use of a liquid as the heat transfer medium and thus a solid metallic matrix is necessary.
Focusing solely on magnetoresistive materials, U.S. Pat. No. 5,767,673 to Batlogg et al. describes an improved magnetoresistance obtained in a thin single crystal perovskite La2/3Ca1/3MnO3 at extremely low fields when two magnetically-soft ferromagnetic (Mn,Zn)Fe2O4 bars were used in close proximity to the perovskite manganite. It is stated that the magnetically-soft material can be thinly layered on the magnetoresistive core or it can be mixed with a magnetoresistive material to produce the improved magnetoresistive element. In each case, it is disclosed that the magnetically-soft material placed in close proximity to the magnetoresistive core serves to increase the magnetic field experienced by the magnetoresistive core resulting in an increased magnetoresistive effect at low applied magnetic fields.
While such advances have been made in the field of magnetoresistance, it is clear that innovative material design and engineering is needed to lower the applied magnetic field necessary to realize the optimum MCE in magnetocaloric materials. In particular, there is a great motivation to bridge the gap between giant magnetocaloric materials and the state-of-the-art AMR permanent magnet refrigerator design.